Most conventional motor vehicle closure assemblies are provided with a seal structure or weather strip intended to prevent the unwanted ingress of rain, wind, dust, and the like from the exterior of the vehicle. The weather strip prevents rain and other weather phenomena from entering the vehicle by either blocking it outright, or by preventing a portion thereof from entering and rerouting the remainder. A secondary goal of most seal structures is to prevent unintentional evacuation of interior air (e.g., heating and air conditioning), and mitigating occupant perceptible exterior vehicle noise.
The door weather strip is generally designed with a base portion for attachment to an outer periphery of the door assembly frame, and a sealing portion projecting integrally from the base portion. When the door assembly is closed or pressed shut, an internally-projecting wall of the sealing portion is pressed or abutted against a contact surface formed around a peripheral edge of the door opening in the vehicle body. The elastic nature of the weather strip and pressure between the sealing portion and contact surface effect a seal between the door opening and the door assembly. As such, structural degradation of the weather strip throughout its operational life and insufficient closing forces (i.e., low closing efforts) negatively affect the adequacy of the seal.
Most current seal structures utilized for sealing opposing surfaces, such as vehicle door assemblies, are generally passive; that is, other than innate changes in modulus of the seal material due to environmental stimuli, the stiffness and cross-sectional geometries are unchangeable. Active materials include those compositions having certain properties, such as stiffness, shape, and dimension, that can be selectively altered through the introduction of an external stimulus. The external stimuli, more commonly referred to as an “activation signal”, may include, but is not limited to, external stresses, temperature, moisture, and pH changes, and electric or magnetic fields, depending upon the type of active material.
Shape memory materials, sometimes referred to in the art as smart materials, refer to materials or compositions that have the ability to “remember” their original shape, which can subsequently be “recalled” by applying an external stimulus (i.e., an activation signal). As such, deformation of the shape memory material from the original shape can be a temporary condition. These capabilities are due, in part, to a temperature-dependent martensitic phase transformation from a low-symmetry to a highly symmetric crystallographic structure. These crystal structures are known as martensite (at lower temperatures) and austenite (at higher temperatures). Shape memory materials such as shape memory alloys (SMAs) and shape memory polymers (SMPs) represent a class of thermally-activated smart materials (TASMs) that undergo a reversible phase transformation responsible for stress-induced and temperature-induced recoverable deformation behavior.